Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device.
Typically, an LED chip includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact is formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer). In other embodiments, a substrate need not be included.
It is known to enclose an LED chip in a package that can perform a number of functions and provide a number of benefits. For example, an LED package can provide mechanical support and environmental protection for the chip, as well as providing electrical leads for connecting the chip to an external circuit, and heatsinks for efficient heat extraction from the chip. An LED package can also perform optical functions. For example, an LED package can include optical materials and/or structures, such as lenses, reflectors, light scattering layers, etc., that can direct light output by the semiconductor chip in a desired manner.
In a typical LED package 10 illustrated in FIG. 1, an LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wirebonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16 containing a wavelength conversion material such as phosphor particles. The entire assembly may then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12.
It is often desirable to incorporate phosphor into an LED package to enhance the emitted radiation in a particular wavelength and/or to convert at least some of the radiation to another wavelength. In general, light is emitted by a phosphor when a photon having energy higher than a bandgap of the phosphor material passes through the phosphor and is absorbed. When the photon is absorbed, an electronic carrier in the phosphor is stimulated from a resting state to an excited state. When the electronic carrier decays back to a resting state, a photon can be emitted by the phosphor. However, the emitted photon may have an energy that is less than the energy of the absorbed photon. Thus, the emitted photon may have a wavelength that is longer than the absorbed photon.
The term “phosphor” is used herein to refer to any materials that absorb light at one wavelength and re-emit light at a different wavelength, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Accordingly, the term “phosphor” is used herein to refer to materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphor particles absorb light having shorter wavelengths and re-emit light having longer wavelengths. As such, some or all of the light emitted by the LED chip at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a single blue emitting LED chip may be surrounded with a yellow phosphor, such as cerium-doped yttrium aluminum garnet (YAG). The resulting light, which is a combination of blue light and yellow light, may appear white to an observer.
Typically, phosphor particles are randomly distributed within the matrix of encapsulant material. Some or all of the light emitted by the LED chip at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a blue-emitting chip may be encapsulated with an encapsulant matrix including a yellow-emitting phosphor. The combination of blue light (from the chip) with yellow light (from the phosphor) may produce a light that appears white. Some red-emitting phosphor particles may be included in the encapsulant matrix to improve the color rendering properties of the light, i.e. to make the light appear more “warm.” Similarly, a UV-emitting chip may be encapsulated with an encapsulant material including phosphor particles that individually emit red, green and blue light upon excitation by UV light. The resulting light, which is a combination of red, green and blue light, may appear white and may have good color rendering properties.
It is important to control the junction temperature of phosphor converted LEDs in order to provide a long life for LEDs. In conventional LEDs, the distribution of phosphor particles in a coating on an LED chip is essentially uniform. FIG. 2 illustrates a conventional LED 20 that includes a chip 12 and a coating of phosphor-containing material 22, such as silicone, on the light emitting surface 12a of the chip 12. Because of the generally uniform distribution of the phosphor particles P in the silicone matrix 22, the average distance that heat generated by the Stokes loss has to travel is approximately half the coating thickness, for example 25-50 μm, depending on the target color point. Arrows A1 graphically illustrate the different distances that heat travels from the various phosphor particles P to the chip 12. Due to the low thermal conductivity of the silicone matrix 22, heat from the down-conversion event may not be easily dissipated, causing the phosphor particles P to heat up and reduce their efficiency. Self-heating of the phosphor particles P, along with poor heat conduction through the coating material 22, causes LED chip efficiency to drop with increasing flux density, as illustrated in FIG. 3. Moreover, the effects become more pronounced as the thickness of the phosphor-containing material 22 increases.